Your crown is located in the back center area of your head. It's exact position tends to vary from person to person. For persons with low cuts, the crown is usually characterized by its unruliness and it must be trained/tamed in order to improve your 360 wave appearance. Although many wavers say use a soft brush for the crown, I have developed my own theory that is based on the contrary. More specifically, I think a hard brush is a better option for developing and maintaining a tightly waved up crown.
The first thing you should do is find the center of your crown. This can be accomplished by using a hand held mirror that will allow you to see your crown in a wall mounted mirror or something similiar. As you look at the center of your crown, imagine it as a traditional analog clock. Place your hard brush parallel to the center of your crown and brush outward toward what would be 12 o'clock. Make sure that you only brush a few inches outward. This will concentrate your brushing action to the specific area of interest. Do this several times, then rotate your brush 45 degrees clockwise and repeat. If you are rotating your brush correctly, then the third rotation will place your brush perpendicular to its starting position. Keep rotating your brush 45 degrees, while brushing outward, until you have covered your whole crown. It is important that you start the brushing motion as close to the center of your crown as possible after you complete each brush rotation. Your brush should rotate 8 times because 360, which is the wave pattern, divided by 45, which is the degrees of rotation, equals 8. Although it isn't mandatory, I recommend rotating your brush clockwise, because most crowns tend to spin out to the right. Make sure that your barber cuts your crown similiar to the way you are training it. Also, keep in mind, that the minimum wavelength for your crown is usually less than the rest of your hair. Therefore, it should be cut slightly higher to increase wave visibility and definition.
Finally, the reason I recommend a hard brush is because the crown has no set grain pattern. Since you're trying to train the crown's hair in a set 360 pattern and help it develop an appropiate grain pattern/angle, a hard brush is essential. If you use a standard nine inch hard brush, you can use a seven inch mini hard brush to increase brushing concentration accuracy. Club and military hard brushes are also effective.